This invention relates generally to electrophotographic printing and more particularly relates to the heat and pressure fixing of particulate thermoplastic particles such as toner by direct contact with a heated fusing member. More specifically, the present invention relates to a release agent management (RAM) system for use with a heat and pressure fuser,
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is provided with an abhesive material layer, such as a silicone rubber or other low surface energy material, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon.
Oil release fluids such as silicone based oils have been applied to the aforementioned surface of the low surface energy layer to both minimize offsetting and to facilitates stripping. See, for example, U.S. Pat. No. 3,964,431. When the fuser system is one which provides for applying silicone oil to silicone rubber or "Viton" a low viscosity silicone oil (i.e. on the order of 100-1000 cs) has most commonly been employed. Donor roll RAM (release agent management) systems have been used as part of heat and pressure roll fusers apparatus for some time. Such a RAM system is disclosed in U.S. Pat. No. 4,214,549 issued on Jul. 29, 1980 to Rabin Moser. This patent illustrates a heat and pressure roll fusing apparatus for fixing toner images to copy substrates wherein the toner comprises a thermoplastic resin. The heated fuser roll is characterized by an outer layer or surface which by way of example is fabricated from a silicone rubber or Viton material to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump from which it is dispensed by means of a metering roll and a donor roll, the former of which contacts the release fluid in the sump and the latter of which contacts the surface of the heated fuser roll.
U.S. Pat. No. 3,716,221 issued on Feb. 13, 1973 to Gorka et al discloses a heat and pressure fuser roll wherein the heated fuser roller includes a fusing roller having a resilient fusing blanket supported on the periphery thereof and heating means to heat the fusing blanket to a temperature sufficient to fuse the particulate material on a copy sheet. A backup roller is urged toward engagement with the deformable fusing blanket to press the receptor sheet carrying the particulate material into contact with the fusing roller. The fuser roller is coated with an offset preventing liquid which is applied thereto from the backup roller at predetermined intervals during operation of the device. The offset preventing liquid is applied to the backup roller via a wick, one end of which is immersed in a quantity of the liquid which is contained in a receptacle. The application of the liquid to the backup roller is controlled such that it is applied once every eleventh revolution of the fuser roller. In other words ten copy sheets are passed through the fuser and then the fuser and backup rollers are rotated an eleventh time without a copy sheet passing therebetween at which time the liquid is applied to the fuser roller via the wick and backup roller.
Compared to wicks, a donor roll RAM provides a significant oil uniformity microscopically on a copy and also to copies for long runs. One major disadvantage of a donor roll RAM system is it creates a major thermal load on the fuser during standby as well as during run. The power going into the RAM system at cold start was determined to be 1000 watts and greater than 200 watts during run. Another disadvantage of the donor roll RAM system is the added UMC (unit manufacturing cost) of the system which in the '549 patent comprises a metering roll, a donor roll and a wiper blade. When a wick is used to apply the liquid to the pressure roller, as in the case of the '221 patent, the liquid is applied as drops or droplets rather than in a thin layer. Not only is it desirable to have a RAM system that uniformly applies the release agent material to the heated fuser roll without draining heat therefrom, it is likewise desirable to reduce the UMC of RAM systems by minimizing the number of components utilized therefor.
U.S. Pat. No. 4,770,116 granted Rabin Moser on Sep. 13, 1988 discloses a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising a thermoplastic resin. The apparatus includes an internally heated fuser roll cooperating with an unheated bare metal backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The heated fuser roll is characterized by a conformable outer layer or surface which by way of example is fabricated from a silicone rubber or "Viton" material to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump and the pressure roll is partially immersed in the fluid. Thus unlike the '549 RAM system, the release fluid is applied to the surface of the internally heated fuser roll via the bare metal pressure roll. The roll structures are such as to provide maximum area of contact in the nip, while minimizing the area of contact between the pressure roll and the copy substrates. In other words, the RAM system does not directly contact the heated fuser roll. Although the problem of the RAM system acting as a heat sink is obviated by the '116 arrangement, it does not provide the attributes of a donor roll RAM system such as that depicted in the '549 patent.
U.S. Pat. No. 4,397,936 granted to Sakata et al on Aug. 9, 1983 discloses a RAM system comprising an applicator roller, supply of silicone and a member for conveying the oil from the supply to the applicator roll on an intermittent basis. The applicator contacts a non-heated fixing roller the equivalent of one revolution thereof for each copy to be fused.
U.S. Pat. No. 4,197,445 discloses a heat and pressure roll fusing apparatus in which an oil applicator roll contacts a heated fuser roll member.
While a donor roll RAM system of the type disclosed in the '549 patent is superior to one that uses a wick, such a system is not devoid of shortcomings. For example, metering blade contamination causes oil streaks, metering blade contamination being the direct result of contamination of the wick utilized in a donor roll RAM system. Wick contamination is caused by the wick collecting clay and toner particles in the space between the wick and the metering roll of the donor roll RAM, and/or wear of the wick surface.